The present invention relates to composite ceramics having high strength at high temperatures and their production process.
So far, composite or engineering ceramics containing silicon nitride as the matrices and spherical particles, plate-form particles, whiskers or fibers of SiC as dispersion phases have been studied. Proceedings of the 23rd Automotive Technology Coordination Meeting, pp. 199-208 refers to engineering ceramics in which the matrices are formed of reaction sintered silicon nitride and the dispersion phases are made up of SiC fibers. Proceedings of the 1st International Symposium on the Science of Engineering Ceramics, pp. 371-376 describes engineering ceramics in which the matrices are formed of sintered silicon nitride and the dispersion phases are made up of plate-shaped particles of SiC. JP-A-62-12760 discloses engineering ceramics in which the matrices are formed of a Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 type of silicon nitride and the dispersion phases are made up of SiC fibers and which are compacted by hot-pressing. JP-A-01-243133 sets forth engineering ceramics in which the matrices are formed of rare earth oxide and silicon nitride and the dispersion phases are made up of plate-form particles of SiC.
JP-P-56-51153, on the other hand, refers to a heat-resistant sintered compact comprising o' and SiAlON phases. The term "o' phase" refers to a solid solution in which the solute Al.sub.2 O.sub.3 fits into the solvent Si.sub.2 N.sub.2 O phase. In addition, Journal of American Ceramic Society, Vol. 74, No. 4, pp. 1095-1097 (1990) shows a heat-resistant sintered body made up of an SiO.sub.2 glass phase and Si.sub.3 N.sub.4.
When the matrix of a conventional composite ceramic material is a reaction sintered type of Si.sub.3 N.sub.4, however, some difficulty is involved in packing it completely, so that it is inferior in terms of strength and acid resistance and so cannot be used at a temperature of higher than 1400.degree. C. When the matrix of a composite ceramic material is Si.sub.3 N.sub.4 containing a sintering aid such as rare earth oxides or Al.sub.2 O.sub.3, it cannot again be used at a temperature of higher than 1400.degree. C., because the grain boundary containing these sintering aids is softened, or the eutectic point of the sintering aids and SiO.sub.2 contained in the Si.sub.3 N.sub.4 material is low, so that a liquid phase can be formed, resulting in a drop of its heat resistance.
Even a heat-resistant sintered body made up of the o' and SiAlON phases that are not composite ceramic materials, when containing an Al component, undergoes a drop of its strength at 1400.degree. C. or higher due to the softening of the grain boundary phase, etc. In another heat-resistant sintered body constructed from the SiO.sub.2 glass phase and Si.sub.3 N.sub.4, the SiO.sub.2 glass phase is unstable at high temperatures. Further, these sintered parts that are not composite ceramics can hardly be used as structural material, because they have no microstructure comprising a columnar crystal characteristic of the sintered silicon nitride part and a grain boundary phase and so have low toughness.
Thus, an object of this invention is to provide composite or engineering ceramics that are stable and have a high strength in the temperature range of room temperature to as high as 1600.degree. C. and possess a toughness high-enough to be usable as structural materials and a process for producing them.